The mouth conducts by an oesophagus into a distinct stomach, surrounded by a well-developed granular liver. The intestine has a "neural flexure," and "either ends blindly in the middle line, or else terminates in a distinct anus between the pallial lobes" (Huxley).
Within the pallial lobes there is a remarkable system of more or less branched excretory tubes, anastomosing with one another, and ending in caecal extremities. This, which has been termed by Huxley the "atrial system," communicates with the perivisceral cavity by means of two or four organs which are called " pseudo-hearts," and which were at one time supposed to be true hearts, but which are now known to be connected with reproduction.
Fig. 204. - Terebratula(Waldheimia) flavescens. A, The shell viewed from behind, showing the dorsal valve, and the perforated summit of the ventral valve above it. B, Inner view of the dorsal valve, showing the shelly loop (l) which supports the spiral arms. C, Inner view of the ventral valve, showing the foramen or aperture (f) in the beak, through which the muscular stalk of attachment passes. D, Longitudinal and vertical section of the animal, showing the spiral arms (a), the stomach (s), and the liver (h). At f is the opening in the beak, with the stalk of attachment (p) passing through it. (After Davidson and Owen.) Some details have been omitted in figs. B, C, and D, for the sake of clearness.
"Each pseudo-heart is divided into a narrow, elongated, external portion (the so-called ' ventricle'), which communicates, as Dr Hancock has proved, by a small apical aperture, with the pallial cavity; and a broad, funnel-shaped, inner division (the so-called 'auricle') communicating, on the one hand, by a constricted neck, with the so-called 'ventricle;' and, on the other, by a wide patent mouth, with a chamber which occupies most of the cavity of the body proper, and sends more or less branched diverticula into the pallial lobes (Huxley). This system of the atrial canals has been looked upon as a rudimentary respiratory apparatus; but its function is more probably to act as an excretory organ, and it certainly serves also to convey away the reproductive elements, the organs for which are developed in various parts of its walls. By Rolleston the pseudo-hearts are looked upon as corresponding with the so-called " organ of Bojanus" of the Lamellibranchiata.
The function of respiration is probably performed, mainly, if not entirely, by the cirriferous oral arms, as it appears chiefly to be by the homologous tentacular crown of the Polyzoa. A unilocular heart is present in some, but apparently not in all, of the Brachiopoda; and the circulation seems to be mainly carried on through the interstices between the tissues.
Fig. 205. - Development of Terebratulina septentrionalis (after Morse). A, Ciliated embryo. B, More advanced embryo, showing commencing segmentation and a rudimentary peduncle (p). C, D, E, F, Further stages of the same embryo. G, Advanced embryo, with a very long peduncle (p), and a circular oral crown of cirri (c). H, Interior of dorsal valve, showing the circular crown of cirri, and the intestine (i). I, Another larva, at the same stage, having the valves opened, and viewed from one side. J, Part of a larva still further advanced, showing the now horse-shoe-shaped crown of cirri; p Peduncle ; v Ventral valve of shell; d Dorsal valve ; c Crown of oral cirri; i Intestine; s Setae springing from the edge of the mantle; / Loop of dorsal valve. (All the figures are highly magnified.)
The nervous system consists of a principal ganglion of no great size, placed in the re-entering angle between the gullet and the rectum. In those Brachiopods in which the valves of the shell are united by a hinge, the nervous system attains a greater development, and consists of a gangliated oesophageal collar.
The sexes in the Brachiopoda appear to be ordinarily distinct, but in some forms they are asserted to be united in the same individual. As regards the process of development in the class, we may take as a type Terebratulina septentrionalis, the metamorphoses of which have been most ably worked out by Professor Morse. In this form, the earliest embryo is a ciliated planula (fig. 205, A), which swims about actively, and soon (B) exhibits a division into three regions or segments, which rapidly become more conspicuous (C). Of these segments, the most inferior (p) becomes the future peduncle, and serves to attach the embryo to some foreign body (D). The middle segment then enlarges, and partially encloses the anterior segment (E and F), the latter ultimately being withdrawn entirely within the former, which becomes converted into the shell-secreting pallial lobes. Next the arms begin to bud out of the sides of the mouth (G), forming at first a circular crown of cirri (c), which forcibly calls to mind the orbicular lophophore of the Gymnolsematous Polyzoa. The peduncle, at first long (as in Lingula), becomes rapidly shorter (I), and the oral crown of tentacles becomes distinctly horse-shoe-shaped (J), thus strikingly resembling the similarly-shaped lophophore of the " Hippocrepian " Polyzoa, The cir-rated " arms " of the adult are finally produced by the growth and development of the free end of the horse-shoe.